1. Field of the Invention
The present invention generally relates to a method for forming a semiconductor device, and more particularly to a method for performing lithographic process to a multi-layered photoresist layer.
2. Description of the Prior Art
For an introduction into semiconductor devices, the size of semiconductor devices have been continuously shrinking, resulting in a smaller semiconductor chip size with increased device density on the chip. The scale of the device substantially depends on the optical resolution of the photolithography process. The resolution is proportional to the wavelength of the exposure and inverse to the numerical aperture(NA) of the exposure device; i.e., R=k1 (xcex/NA), that means a better resolution analysis can be abstained by using a light source with a short wavelength or exposure device with large a numerical aperture (NA).
Because depth of focus (DOF)=k2 (xcex/NA2), When a light source with short wavelength or lens with large numerical aperture (NA) is used, depth of focus (DOF) is unacceptable. Therefore, it is the most important critical point in photolithography that how to use resolution in combination with depth of focus (DOF) to obtain the preferred process conditions.
With the increase process complexity, substrate of the wafer upon which the pattern is imaged is typically non-planar, primarily because of non-uniformities in the starting wafer surface and topographies creased by deposition and etching steps during processing. However, as the plane of image departs from the plane of optimal focus in the resist, the image quality deteriorates. Therefor, it is another important detail to be solved in photolithography. That pattern needs to be imaged on non-planer surface of a photoresist layer.
For the forgoing reasons, there is a need for a method that will increase both resolution and depth of focus (DOF) in photolithography is necessary.
In accordance with the present invention, a method is provided for performing lithographic process to a multi-layered photoresist layer that substantially can be used to increase resolution in conventional process.
One object of the present invention is to provide a method for performing lithographic process to a multi-layered photoresist layer to increase resolution.
Another object of the present invention is to provide a method for performing lithographic process to a multi-layered photoresist layer to increase process depth of focus window.
In order to achieve the above object, the present invention provides a method for performing a lithographic process to a multi-layered photoresist layer. The method at least includes the following steps. First of all, a substrate is provided. Then, the first photoresist film is formed on the substrate, and a mask layer is formed on the first photoresist film, wherein the mask layer is different from the first photoresist film. Next, a second photoresist film is formed on the mask layer, and a pattern is transferred into the second photoresist film by using a lithographic process. Then, the mask layer is etched by using the second photoresist film as a first mask such that the pattern is transferred into a mask layer. Finally, the pattern is transferred into the first photoresist film by using the mask layer as a second mask.